This invention relates to a feeder system for trolley-assisted dump trucks, and more particularly to an alternating current feeder system for use in a feeder circuit of a.c. system for trolley-assisted dump trucks.
In view of the availability of petroleum becoming tight increasingly in the late years, there is a relevant tendency towards energy saving even at large-scale mines. In particular, in the district where ample power can be generated by hydraulic power and by thermal power obtained by burning coals, but is lacking in petroleum resources, there can be seen a strong demand for electrization of mining machinery with a view to saving petroleum resources.
In addition to the petroleum saving tendency, there can be seen a tendency towards reduced productivity due to the suspension of movements of vehicles so as to prevent a pit of the mine from oxygen deficient condition arising from the exhaust gas smog emitted from the engines of ore carrying trucks and the amount of which increases as the pit becomes deeper.
In order to eliminate such difficulties of late years, there has been proposed and emloyed a double-wire trolley-assisted dump truck system making use of the cheap commercial power supply in the ascent courses from the bottom of the pit to a dumping area.
In a feeder circuit for trolley-assist, the power for operation is supplied from a substation to one of trolley wires, and then it will pass through current collectors so as to move the running motors of electrically driven dump trucks, and through the other of trolley wires, back to the substation again.
This feeder circuit is classified into two system, namely, a direct current system and an alternating current system. In the case the alternating current system is employed, there is no need of provision of transformers in the substantion, and so the equipment is simplified. However, when an alternating current feeder system is employed in the single phase trolley-assist system, the single phase load is taken from the three phase power supply network so as to cause an unbalanced voltage on the side of three-phase, thus causing in extreme cases temperature rises in the induction motors and generators connected with the three phase power supply network system and giving other bad influences to them.
It is well known that if a Scott connection is used in the transformer to convert three-phase into single phase to prevent such troubles, then a balanced single phase direct current voltage is induced on the secondary circuit, even if a balanced three phase alternating current is supplied in the primary feeder circuit.
In case the Scott transformer is used, the phase difference between the first circuit and the secondary circuit is 90 degrees, and so it is necessary to provide a non-voltage applied section in the feeder circuit, which section being referred to
If the different phase sections are provided on the ascents in the pit of mine, there have occurred troubles wherein when a dump truck passes from the first feeder section through the non-voltage applied section, its running speed will reduce so that it is necessary to allow high electric current flow through its motor when the truck has entered the second feeder section thus causing heat generation and fusion of trolley wires and abnormal wear of pantagraphs and sliders, and also because there is a phase difference of 90 degrees between the first feeder section and the second feeder section, if the dump truck under current collecting condition is allowed to enter the different phase section, then a big arc will occur thereby damaging the pantagraphs and sections.